Lesson 02 · Register pairs - Build a Game Boy Emulator

Today you teach the CPU to treat its register pairs - BC, DE, HL, and AF - as single 16-bit values instead of two separate bytes. Every address the CPU touches is 16 bits wide, so these combined views are how you will point at memory for the rest of the project.

The goal

Let each register pair (BC, DE, HL, AF) be read and written as a single 16-bit value.

Start here - the target

TO DO

Scenario: Each register pair combines and splits its two bytes

GivenB is 0x12 and C is 0x34

Whenthe pair BC is read

ThenBC is 0x1234

Andfor each pair BC, DE, HL, AF - setting it to 0xABCD makes the high register 0xAB and the low 0xCD

Background

Addresses on the Game Boy are 16 bits wide, but the registers are only 8. The CPU bridges the gap by pairing registers: B and C together form BC, and likewise DE, HL, and AF. The first register of each pair holds the high byte, the second the low byte - so B=0x12, C=0x34 reads as 0x1234.

HL is the workhorse pair used to point at memory, which you will lean on constantly once instructions start reading and writing RAM. Build the four combined views now - read and write both directions - and the rest of the CPU gets a clean 16-bit handle whenever it needs an address.

Make it work

// high register is the top byte, low register the bottom byte
func (r *Registers) BC() uint16 {
    return uint16(r.B)<<8 | uint16(r.C)
}
func (r *Registers) SetBC(v uint16) {
    r.B = uint8(v >> 8)
    r.C = uint8(v)
}
// DE, HL, and AF follow the identical pattern - a table-driven test covers all four